1. Field of the Invention
The present invention relates to thermal treatment equipment and a thermal treatment method for heating a substrate made of glass, ceramic or the like, and to a manufacturing method of an image display apparatus.
2. Related Background Art
Conventionally, image display apparatus include an image display apparatus for displaying an image on a screen by radiating electron beams on a phosphor, in addition to a liquid crystal display and a plasma display. The image display apparatus using the electron beams includes a face plate of a substrate on which the phosphor is formed, a back plate of a substrate on which surface conduction electron-emitting devices are formed as electron sources for generating the electron beams, and an outer frame for keeping a depressurized atmosphere between the two substrates. The image display apparatus using the electron beams accelerates and radiates the electron beams from the surface conduction electron-emitting devices to the phosphor in the depressurized atmosphere to make the phosphor emit light, and thereby the image display apparatus forms an image (see, for example, Japanese Patent Application Laid-Open No. 2000-251716). Incidentally, some image display apparatus are severally provided with thin plate-like spacers between the two substrates lest the two substrates should be distorted at the time of depressurizing the space enclosed by the face plate, the back plate and the outer frame.
Thermal treatment at a manufacturing process of the image display apparatus using the electron beams includes bake treatment for eliminating chemically absorbed water, in which water molecules are polarized to be mutually bonded with each other, from the surfaces of the substrates in addition to the treatment of fixing the face plate and the back plate with the outer frame put between them. When the face plate and the back plate are tried to be assembled in the depressurized atmosphere without performing the bake treatment, the pressure does not readily lower owing to the degassing of the chemical absorbed water, and then it takes a long time to reach a target degree of vacuum for assembling the face plate and the back plate.
If the bake treatment is performed to a back plate after the electric sources, wiring and the like have been formed thereon, or to a face plate after wiring and the phosphor have been formed thereon, it is better that the treatment time is shorter also for reducing a load of the thermal treatment to the devices and the wiring formed thereon. Moreover, because the bake treatment is performed a plurality of times at a manufacturing process, the treatment time at a time is desired to be shorter also for shortening a manufacturing period.
A thermal treatment apparatus for the bake treatment is equipped with a hot plate composed of a heating wire put between insulating materials in a treating chamber, and heats a substrate placed on the hot plate from the back surface of the substrate.
There has been a problem in which the temperature of a substrate does not readily rise at the time of the bake treatment because a hot plate having a large thermal capacity is used for the heating of the substrate in the thermal treatment equipment to be used for the bake treatment. In the case where a heating rate is set to be large for raising the temperature of the substrate rapidly when the bake treatment is performed after the spacers have been formed on the surface of the substrate of the back plate, the temperatures of the spacers on the surface of the substrate rise faster than the temperature of the substrate because the spacers have smaller thermal capacity in comparison with that of the substrate. Consequently, the spacers sometimes are broken owing to the temperature differences between the spacers and the substrate. Moreover, the temperature distribution of the substrate sometimes becomes uneven and the substrate sometimes becomes warped. On the other hand, when the heating rate is set to be slow for making the temperature distribution even, a problem in which the treatment time becomes longer is generated.